Hi there!
Sorry for your issue.
The home page top area holds an empty section and the section gets default top, bottom padding values. It pushes the header below and as the header is transperant so it seems like white. Here is a screenshot for your ease:
https://prnt.sc/lS9_1BTvPGnf
You can remove that section from your page or you can use the following code in your custom CSS:
.sp-page-builder .page-content #section-id-1635717663436 {
display: none;
}
Note: Path towards custom CSS(Joomla 3): Extensions>Templates>Styles> Click on your current default template name then choose Template Options. Then find custom CSS within the custom code.
For your other issue, I have checked your starting page but could not find that portion you mentioned you edited last time and then problem started. Would you kindly have a look if it is possible to let me know which addon it was? It will help me to assess the issue I think.
Best Regards